Analysis of the effect of cerium on the formation of non-metallic inclusions in low-carbon steel

The research focused on the influence of cerium on the chemical composition and morphology of non-metallic inclusions in pre-oxidised steel to which Al, Ca, and Ce was added in different amount and order. Calculations were carried out using our own computer program. The simulation results obtained according to two calculation models helped identify precipitates from the Ce–O–S system. The possibility of CeN formation was also identified. The trace amounts of these inclusions were also found in the results. Consideration of the physicochemical phenomena at the boundary, as well as the interfacial partitioning and the sulfur partition coefficient, influences the favourable chemical composition of the inclusions, limiting it mainly to compounds from the Al2O3, Ce2O3, and CaS systems. It was found that the addition of Ce before Ca causes the elimination of MnS precipitates and Ca-containing inclusions in the steel.

The effect of cerium in steel is very complex because, in addition to its role as a deoxidizing and desulfurizing material, it interferes with the structure of inclusions and their chemical composition, morphology, and properties, consequently changing their plastic formability and corrosion resistance of steel. Mingya et al. 8 presents studies on interfacial tension measurements of particles: SiO 2 , Al 2 O 3, and CeS in liquid steel. Low values of interfacial tension were found for CeS precipitates. This means that the presence of CeS in the liquid phase lowers the energy barrier for the formation of solid phase crystals several times. In high-sulfur steels, the authors observed that cerium sulfides act as crystallization centers, causing grain fragmentation and the formation of a zone of equiaxial crystals. On the other hand, in low-sulfur steels, the compound Ce 2 O 3 was found, which can also act as a center for heterogeneous nucleation of other phases [12][13][14][15][16] . This was also confirmed by the research conducted by Adabavazeh et al. 17 , who found that Ce 2 O 3 can act as nucleation sites for IAF. The study showed that the separation of cerium oxide, which has a high melting point, also affects the grain size in ultra-low carbon steels. It was found that the yield point significantly improved and the structure became fine-grained as a result of the increase in the number of nucleation centers of the solid phases. www.nature.com/scientificreports/ Consequently, controlling the formation of desirable cerium-based inclusions favors higher strength and hardness of steel [12][13][14][15][16] . The fine-dispersed phase of Ce-containing precipitates can be used as centers for heterogeneous nucleation of sulfides, nitrides, or carbides, and the resulting cerium-containing precipitates inhibit grain growth and affect the formation of ferrite from austenite. As a result, they improve the mechanical properties, weldability, and fatigue strength, as well as corrosion resistance [18][19][20] .
In this work, authors investigated the behavior of cerium and its impact on the formation of non-metallic inclusions in steel using a computer program, the operation of which was described in previous work [21][22][23][24][25][26][27] .

Materials and methods
The computer program WYK_STAL 25 was used for making a real-time simulation of the refining process. The algorithm used in this program is presented in works 22,25 . The results of calculations carried out with the WYK_ STAL program are comparable to those of industrial smelting. This is possible by matching such parameters as the size of the ladle, the method of mixing, and the mixing ratio. In the case of deoxygenation and desulfurization, hydrodynamic conditions are of particular importance. The transport of reactants consists of several stages. First, the reactant dissolves in the volume of metal and is distributed in it. Secondly, it diffuses through the boundary layer, so the concentration of reactants changes from average to equilibrium values corresponding to the equilibrium of a given reaction for the precipitate formation. For the purposes of the simulation, the mixing ratio has been determined on the basis of a thorough analysis of the smelting processes of 10 steels that are refined in a similar way. With the program, it is possible to control the dosage of alloying additives similarly to the real metallurgical process, where the process can take place continuously, in doses, or in a single dose. The program uses several calculation modules with such options as deoxidation, desulfurization, nitride formation, and verification of results. It is used to calculate the actual mass and chemical composition of the reacting phases at any point in the process, with the change in reactant concentration represented by a 1st-order kinetic equation. The WYK_STAL computer program has internal databases, physical and chemical parameters for steel and slag. The original version of the program was supplemented with the data given in Tables 1 and 2.
The calculation of the formation of non-metallic inclusions in liquid steel can be performed in several variants depending on the type of model used and taking into account the properties of non-metallic inclusions. Variant 1 uses a model which assumes that the activity of the inclusion a = 1, in variant 2 the activity coefficient of metallic components f = 1, and lastly, variant 3 takes into account the metal-slag interfacial partition coefficient 25 . The set refining time was 30 min, the melt weight 140 Mg, the temperature of the liquid steel in the ladle 1670 °C, and the pressure 1 atm. The steel, with a chemical composition of 0.054% C, 0.05% Mn, 0.23% Si, 0.007% P, 0.005% N, and 0.01% S, was pre-oxidized hence the dissolved oxygen content of 0.01%. The slag in the ladle consisted of the following compounds: CaO-45%; Al 2 O 3 -2%; MgO-9%; MnO-5%; SiO 2 -12%; FeO-27%; this composition was supplemented by 100 kg of SiO 2 in the 2nd minute of refining. Laboratory smelting was performed in a vacuum furnace and 2.2 g. Cerium was added to 1000 g of the pre-oxidized steel. Samples taken from the ingot were analyzed using a Tescan model VEGA3 W-REM scanning microscope equipped with a Bruker Nano XFlash 610 M EDS detector.  Fig. 2. The addition of aluminum caused the steel to deoxygenate to a level of 1 ppm, the introduction of calcium at the 10th minute of the process caused the sulfur content to drop to a level of 0.001% S, cerium, which was introduced at the 20th minute of refining, dissolved into the steel and its final content was at 0.0527%. With this order of dosing additives, in the first phase of the process, Al 2 O 3 inclusions are formed, the addition of calcium in the 10th minute of the process causes the formation of CaO and CaS precipitates. Calcium reacts with Al 2 O 3 and as a result, CaO is formed. From the course of the line of change in sulfur content (Fig. 2), it is clear that it was consumed for the formation of CaS. The addition of cerium in the 20th minute of the process changes the percentage in the inclusions. Cerium has a higher chemical affinity for oxygen than aluminum, so it will modify Al 2 O 3 inclusions, and CeAlO 3 -type inclusions may form as a consequence.   (Table 1). As a consequence, the aluminum content in the steel is 0.013%. 10 min after the addition of cerium, the oxygen and sulfur content decreases.
Changing the order in which the additives are dosed results in an increased expenditure of cerium consumed during the formation of non-metallic inclusions. The results shown in Fig. 4 are interesting as the Ca addition at 20 min of the process results in its dissolution in steel and no inclusions are formed. It is also significant that the final content of cerium in steel is 0.05%, while for variant 1 in Fig. 2, it totals to 0.009%. The loss of cerium is caused by the formation of cerium sulfide and oxide inclusions, accompanied by a simultaneous increase of calcium in steel. In Fig. 5, the steel after refining is dominated by CeS inclusions, while in Fig. 3      When comparing the results of calculation models for variants in Figs. 4 and 8, attention is paid to the higher final Ce content in the steel. In contrast to the results given in Fig. 4, the introduction of calcium at the 20th minute of the process results in an increase in the Ce content of steel from 0.013% to slightly over 0.015%. This means that the loss of cerium is considerably lower than that resulting from calculations using variant 1.
In the case of non-metallic inclusions, the choice of calculation model also influences the percentage of individual inclusions in the steel. This is particularly true for CaS inclusions, which account for nearly their 20% presence in steel (Fig. 9), while for variant 1 (Fig. 5) their content is below 3%. The opposite is true for CeS, where the final content is below 10% (Fig. 9) and over 35% for the variant shown in Fig. 5.
There is therefore a direct correlation between sulfur and oxygen content as far as the formation of nonmetallic inclusions in steel is concerned. As the steel was initially deoxidised and then further deoxidised with Al (Al 2 O 3 was produced), the S/O ratio was high. This means that the addition of cerium at the 10th minute of refining results in the formation of cerium sulfide and can cause modification of the Al2O3 inclusions and the reactions (see reaction 3 and 5 in Table 1).
However, the formation of cerium oxysulfide is unlikely due to the low oxygen level of the pre-oxidised steel at the time of cerium dosage. Traces of CeN were identified in all analysed variants (Fig. 10).

Microscopic analyses.
The smelting was carried out in an induction furnace with a 0.13 Pa vacuum. No alloying or deoxidising additives other than cerium were introduced during the process. The steel was then cast into a mould in an argon atmosphere, and samples were taken from various areas of the ingot. The essential examination by scanning electron microscopy was preceded by observation by optical microscopy. Numerous spherical, fine precipitates were identified in the samples (Fig. 11).